In NDIR measurements the wavelength range is selected to cover the maximum absorption band in the gas to be measured. Absorption causes loss in transmitted intensity of radiation, thus offering a method for determination of gas concentration. Typically, the radiation source is an incandescent lamp and the desired wavelength range is selected with the help of a filter. The method is handicapped by measurement errors caused by intensity variations in the lamp output and contamination of the optical channel. Furthermore, changes in detector sensitivity may reduce measurement accuracy.
Due to the instability of an NDIR measurement system, the equipment must be calibrated frequently with the help of calibration gases. An alternative method is the use of a reference channel which permits separate monitoring of the input radiation intensity to the actual measurement channel containing the gas under measurement.
A prior-art embodiment employs in the reference channel a filter whose bandpass wavelength is selected so that the measurement is performed at wavelength corresponding to an absorption minimum in spectrum of the radiation transmitted through the gas. Thus, the value of transmitted intensity measured at the reference wavelength can be compared with the value measured as an absorption band of the gas. The method necessitates the use of two separate filters, and the shifting of the filters makes the system mechanically complicated.
The U.S. Pat. No. 4,709,150 discloses an embodiment in which the prevention of the optical channel contamination is attempted through making the measurement channel from a porous material that is easily penetrable for the gas to be measured, while being capable of preventing major contamination components of ambient air from entering the measurement channel. The porous material can be made from an appropriate plastic material or porous stainless steel. The method fails, however, to remove measurement error caused by a decline of lamp output intensity.
The U.S. Pat. No. 4,500,207 describes a measurement apparatus and method in which the internal gas pressure of the measurement channel is modulated at constant temperature with the help of a movable membrane placed on the wall of the measurement channel. Pressure modulation in an NDIR measurement varies the density of the gas under measurement, whereby additional information can be gathered from the gas to be measured that can be used for eliminating instability caused by lamp aging and contamination. Although the movable membrane can be constructed in the same manner as a loudspeaker, the system becomes mechanically complicated. Another disadvantage is that, to implement the gas pressure modulation, the measurement channel must be isolated from the ambient surroundings using a channel of slow gas diffusion. This causes the system to have a relatively long response time to changes in the ambient gas concentration.